The present invention generally relates to the production of steel tubing. More particularly, this invention relates to a process of producing high pressure tubing of austenitic stainless steels using a high frequency induction welding process.
In typical high frequency tube welding processes, a strip of mild steel is passed through several sets of steel rolls to form the strip into a tubular shape having a longitudinal open seam, which is then passed through a high frequency induction coil that welds the seam to produce the desired tube shape. The induction coil generates eddy currents in the tubular shape, with the induced current density being highest at the edges of the seam to cause rapid heating of the edges. Thereafter, pressure (weld) rolls press the edges of the seam together to form a butt weld joint. The efficiency of the welding process is improved by positioning a ferrite rod (impeder) within the tube at the welding location to reduce the current flowing around the inside surface of the tubular shape and thereby promote current flow along seam edges.
While induction welding methods have worked well for producing mild steel tubing, difficulties can be encountered when attempting to produce austenitic stainless steel tubing for high pressure applications, such as coolant tubes for heat exchanger systems. Examples include tubes formed of Type 300 and 900 stainless steels and intended to have tube diameters of about 8 to 26 mm and wall thicknesses of 0.127 to 0.76 mm. Problems encountered include leaks, low strength, and poor corrosion resistance within the weld joint. Consequently, the production of austenitic stainless steel tubing by induction welding processes has typically been limited to decorative or otherwise non-pressure applications, while tungsten inert gas (TIG) welding processes have typically been the process of choice for producing high pressure tubing formed of austenitic stainless steels. However, TIG welding processes generally require relatively low throughput (e.g., about six to thirty fpm (about two to ten meters/minute), as compared to induction welding process that are capable of line speeds of 100 fpm (about 30 meters/minute) and above.